A boundary microphone (on-surface sound pickup microphone) is also called a surface mount microphone because it is used by being installed on a table or floor surface in a TV studio, a conference room, or the like. As described in Patent Document 1 (Japanese Utility Model Registration No. 2515812), the boundary microphone uses a flat microphone case.
One example thereof is explained by reference to FIG. 2. FIG. 2A is a sectional view in the lengthwise direction of a boundary microphone, and FIG. 2B is a sectional view in the width direction taken along the line II-II of FIG. 2A.
Referring to FIGS. 2A and 2B, a microphone case 1 used for a boundary microphone is basically made up of two elements: a flat metallic base part 10 the upper surface side of which is open, and a metallic microphone cover 20 having a large number of openings (sound wave introduction holes), which is attached to the base part 10 so as to cover the upper surface of the base part 10.
Usually, the base part 10 is formed by casting such as zinc die casting, and a rear wall part 11 thereof is formed with a cord insertion hole 12. Also, on the bottom surface of the base part 10, a cushioning rubber sheet 13 is provided along the bottom surface.
For the microphone cover 20, a punched plate (perforated plate) is used. In place of the punched plate, a wire net member is sometimes used. The microphone cover 20 is screwed to the base part 10 by using screws, not shown.
As the boundary microphone, a condenser microphone is usually used. The condenser microphone includes a microphone unit serving as a sound pickup section and an output module section (also referred to as a power module section) having a sound signal output circuit, a power supply circuit, an output connector, and the like.
In the case of a large-size boundary microphone, the microphone unit and the output module section are housed in the microphone case thereof. However, in a small-size boundary microphone, as shown in FIG. 2A, only the microphone unit 30 is housed in the microphone case 1, and the microphone unit 30 is connected to the output module section, not shown, which is provided outside the case, via a dedicated microphone cord 31.
In this case, in the microphone unit 30, a field effect transistor (FET) is incorporated as an impedance converter. As shown in FIG. 2B, the microphone unit 30 is fixed to the base part 10 by a fixing band 15, and the unit case on the earth side and the base part 10 are connected electrically to each other.
The microphone cord 31 is inserted through the cord insertion hole 12 via a rubber-made cord bush 14, and a binding band 16 for preventing coming-off is attached to the inside of the cord bush 14 to ensure a predetermined draw-out strength.
The microphone cord 31 includes a power wire 31a for supplying power from the output module section to the microphone unit 30, a signal wire 31b for giving sound signals, which are generated from the FET, to the output module section, and a shielding wire 31c that electrostatically shields the power wire 31a and the signal wire 31b and connects them to the ground. As the microphone cord 31, a two-core shield covered wire in which the outer periphery of the shielding wire 31c is covered with an electrical insulating outer skin 31d is used.
This microphone cord 31 is vulnerable to noise (electromagnetic waves etc.) coming from the outside because the sound signals are transmitted unbalancedly, which poses a problem of generation of noise caused by strong electromagnetic waves emitted from a cellular phone that has come into wide use rapidly in recent years.
When a cellular phone is used, considerably strong electromagnetic waves (for example, within the range of about several centimeters to several tens centimeters, a field intensity reaching tens of thousands times of field intensity produced in the city by commercial electric waves) are generated. If a high-frequency current caused by the electromagnetic waves intrudes into the microphone case 1, the current is detected by the FET incorporated in the microphone unit 30, and therefore noise is generated.
If the electrical connection between the base part 10 and the microphone cover 20 is stable in terms of high frequency, a big problem does not occur even if the microphone is subjected to strong electromagnetic waves. Thereupon, the applicant of the present invention has proposed, as Patent Document 2 (Japanese Patent Laid-Open No. 2005-333180), a technique in which a conductive cloth (conductive fabric) is held between the base part 10 and the microphone cover 20.
According to the invention described in Patent Document 2, an expected effect can be achieved for a shield of the circumference part of the microphone case.
However, in the case where the microphone cord 31 with some degree of length is laid in the microphone case 1 as shown in FIG. 2A, if a cellular phone is used in the vicinity of the microphone case 1, the high-frequency current cause by strong electromagnetic waves coming from the cellular phone flows into the shield of the microphone unit 30 via the shielding wire 31c of the microphone cord 31, and this current is detected by the FET, and therefore noise is sometimes generated.
Accordingly, an object of the present invention is to provide a boundary microphone in which a high-frequency current caused by extraneous electromagnetic waves is prevented from flowing into a microphone case via a microphone cord that is drawn into the microphone case.